The invention relates to parts coated with a protective coating, and to methods of fabricating such parts.
At present, for the hottest parts in turbine engines, only nickel-based superalloys are used on an industrial scale. Although such nickel-based superalloys are coated in a thermal barrier system, their utilization temperature can be limited to 1150° C. because of the proximity of their melting point.
Recent research work has focused on using novel materials based on refractory metals capable of being used at temperatures higher than the utilization temperatures of nickel-based superalloys. These families of materials are commonly referred to as: refractory metal-intermetallic composites (RMICs).
Among the solutions that have been found, niobium-based alloys appear to be particularly promising for replacing, or for being used together with, existing nickel-based superalloys. These various alloys have the advantage of presenting melting points that are higher than those of existing superalloys. Furthermore, niobium-based alloys may also advantageously present densities that are relatively low (6.5 grams per cubic centimeter (g/cm3) to 7 g/cm3, in comparison with 8 g/cm2 to 9 g/cm2 for nickel-based superalloys). Such alloys can thus advantageously serve to reduce significantly the weight of turbine engine parts, e.g. high-pressure turbine blades, because of their low density and their mechanical properties that are close to those of nickel-based superalloys at temperatures close to 1100° C.
In general, niobium-based alloys may include numerous addition elements such as silicon (Si), titanium (Ti), chromium (Cr), aluminum (Al), hafnium (Hf), molybdenum (Mo), or tin (Sn), for example. These alloys present a microstructure constituted by a niobium matrix (Nbss) reinforced by dissolved addition elements in solid solution. This phase provides the alloys with toughness at low temperature. The refractory matrix is associated with precipitates of refractory metal silicides of composition and structure that may vary depending on the addition elements (M3Si, M5Si3).
These alloys can present particularly advantageous mechanical properties at high temperature (T>1100C.°). Nevertheless, their oxidation behavior when hot can at present limit their use on a large scale. Particularly, when niobium silicide based alloys are exposed to high temperature (greater than 1000° C.), they can oxidize by internal oxidation as a result of oxygen diffusing through the alloy (mainly in the niobium solid solution). A layer may then form on the surface that comprises a mixture of oxides coming from elements contained in the substrate. The resulting oxide layer can present low adhesion without any protection because of the anarchic growth of the unwanted oxides. More or less complex silicates may be formed. Without external assistance, the silicon content of the alloys can be insufficient to generate enough silicates to develop an oxide layer that provides sufficient protection during exposure to high temperature.
There therefore exists a need to improve the ability of niobium-based alloys of this type to withstand corrosion and oxidation when hot.
There also exists a need to have new materials that present both good mechanical properties (toughness when cold and creep at high temperature for moving parts) and also good resistance to corrosion and oxidation at high temperature.